I had managed to use one for about a year without causing any damage. The only problem I had was with the HRM. It was fine when it was new, but after like 6 months it sometimes would stop reading the heart rate, showing a "0". I changed the batteries but it didn't work.

I think the sensors are pretty strong. The cadence sensor looks weak there, but I had never had any problems with it. I think it can take some rubbing, but I wouldn't know what would happen if you step on it or something.

It could be dead batteries as Bike Lover said, or may be the sensor is not close enough to the unit. I just checked the manual and it says distance between the unit and the sensor should be less than 50 cm. Also the magnet should be as close as possible to the sensor. I can't really think of anything else.

thanks guys for the info but i dont think its the battery because it displays everything else fine i have the speed censor as close as i can get it too the dispaly, i think under 50cm, and i spin the front wheel fowards and i get no speed read out.

What I actually meant was the sensor battery. If I remember correctly they are replaceable and others have had an issue with the batteries being dead when receiving the unit. That's why I suggested it. If you bought it from a LBS, you might want to take it in and see what it says about it.